Such sensors have wide commercial and industrial application. To illustrate their use and to provide some indication of the significance of the invention the following examples are chosen. The list is by no means exhaustive and is given merely to illustrate a variety of applications of such sensors and to indicate the nature and scope of applicability of the invention.
1. Solid electrolyte ceramic sensors are used widely to monitor the oxygen content of the exhaust gas produced by an internal combustion engine. The sensor output voltage is used to regulate the efficiency of the engine by providing feedback to a device that controls the air-to-fuel ratio. In one type of such sensor, the solid electrolyte has the general shape of a thimble and is comprised of a stablized zirconia material, with platinum electrodes formed on the interior and exterior surfaces of the material. Typically, such a sensor operates at exhaust temperatures above 400.degree. C. and requires some time to heat up before it becomes responsive. An auxiliary electrical heater may be incorporated in the sensor to overcome this limitation. An example of such a sensor, with an auxiliary heating element, is described in U.S. Pat. No. 4,175,019 issued Nov. 20, 1979 to Michael P. Murphy. Automobile sensors of this type are used extensively to reduce exhaust emissions and achieve fuel economy. Their response times and the temperatures at which they operate reliably are important features.
2. Solid electrolyte sensors may be used for the quantitative measurement of oxygen pressure inside a vacuum chamber over the range 1 to 10.sup.-7 Torr. An example of such a device, and its performance as a partial pressure oxygen gauge, has been given by C. J. Mogab, J. Vac. Sci. Technology 10, 852-858 (1973). Such a gauge normally operates at temperatures between 600.degree. and 800.degree. C. The low pressure limit is determined by the permeation of oxygen through the solid electrolyte. This not only alters the pressure that is to be determined, but causes departure of the sensor output voltage from the true value given by the well-known Nernst equation (see below).
3. An important application of electrochemical oxygen sensors is the determination of the concentration of oxygen in molten metals. See, for example, New Application of Oxygen Sensors to Ironmaking and Steelmaking in Japan by K. Kagata et al published in Transactions ISIJ 25, 204-211(1985); and Progress of Chemical Sensors with Solid Electrolytes at High Temperature by K. S. Goto published in Proceedings of International Meeting on Chemical Sensors, Fukuoka, Japan, Sept. 19-22, 1983. Typically, such sensors operate at temperatures in the range 700.degree. to 1600.degree. C., depending on the metal whose oxygen content is to be determined. In devices of this nature the solid electrolyte is often in the form of a pellet that is sealed or embedded into one end of a ceramic or quartz tube. For the determination of oxygen in liquid steel, where temperatures of about 1600.degree. C. and highly corrosive conditions are encountered, such devices are usually operated as disposable probes. The pelleted end of the tube is plunged into the liquid metal and the sensor output voltage is recorded continuously until failure of the probe occurs. The output voltage at the moment of failure is then generally accepted as the true output voltage corresponding to the oxygen content of the liquid metal. Such sensors depend for their reliability on a fast response of the output voltage to rapid changes in temperature.
4. Other major applications of solid electrolyte sensors are in the glass and ceramic industries as, for example, in monitoring the oxygen content of molten glass or in monitoring the partial pressure of oxygen in ceramic kilns to control the color of glazes. They are also used in direct reduction kilns for the production of iron, in copper smelting reverbatory furnaces, and in furnaces for the heat-treatment of metals as, for example, in gas carburizing for the hardening of metal surfaces. They are also used extensively to measure the oxygen content of boiler flue gases. They may be employed as safety devices in which the sensor output voltage is connected to an alarm system to warn of impending explosive mixtures if a combustion process fails.
By constant monitoring and controlling the atmosphere in such processes, considerable savings in fuel can be effected. The location of the probe is often an important consideration. In some applications, for example, it may be desirable to locate the sensor close to a flame, to indicate the partial pressure of oxygen in the combustion gases at that point. In other applications it may be desirable to locate the sensor at a position remote from the source of combustion as, for example, in a flue or stack, to indicate the average partial pressure of oxygen in the products of combustion. The probes should thus be capable of responding accurately over a wide range of temperatures and/or oxygen pressures. Such probes may also have to retain their operating characteristics over periods of months or even years of service and it is important in such cases that the probe should not be susceptible to what is commonly termed aging, i.e. changes in the sensor output voltage over prolonged usage. The time of response of the probe to rapid changes in pressure or partial pressure of oxygen is important in many applications. The passage of oxygen through the probe should be minimal, so that the sensor output voltage corresponds closely to the true value for the oxygen pressure or concentration to be determined.